As hard disk drive apparatuses increase in capacity and reduce in size, highly sensitive and high-resolution thin-film magnetic heads are being demanded. In order to satisfy the demand, giant magentoresistive effect (GMR) thin-film magnetic heads with GMR read head elements each having a multi-layered structure with a magnetization-fixed layer and a magnetization-free layer become widely used. On the other hand, tunnel magnetoresistive effect (TMR) thin-film magnetic heads with TMR read head elements having higher sensitivity and higher resolution are put to practical use.
Nowadays, the magnetoresistive (MR) read heads are widely used in a popular magnetic head because an MR element with higher sensitivity is included therein. A plurality of performance of a magnetic head must be tested before the magnetic head is used, which includes dynamic flying height (DFH) performance, Signal-to-Noise Ratio (SNR) performance, reliability, stability and the like. Specially, as the noise is generated, the performance of the magnetic head is weakened, for example, the flying height of the magnetic head is unstable and hard to control, which cause the stability is reduced and, in turn, the reading performance is weakened.
Thus, a noise testing process must be carried out before the magnetic head product is put into use. One of common methods for determining whether a magnetic head is acceptable or defective by measuring noise generated in response to an external magnetic field. And noise profile can be obtained and a noise level of the magnetic head can be measured out, in turn, an acceptable or defective magnetic head can be estimated and judged.
The following method has been used as a conventional method of testing an MR read head by applying an external magnetic field with a direction the same with that of a longitudinal bias field. As shown in FIG. 1a, the MR read head 10 includes two hard magnets 112, an MR element 114 sandwiched between the two hard magnets 112, and two shielding layers 116 disposed on both sides of the MR element 114 and the hard magnets 112. As shown in FIG. 1b, the MR element 114 includes a first ferromagnetic layer 121, a second ferromagnetic layer 122 and an anti-ferromagnetic (AFM) layer 123 which is formed in physical contact with the second ferromagnetic layer 122 to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer 122 is constrained or maintained by the exchange coupling, thus the second ferromagnetic layer 122 is also called “pinned layer” 122. In general, the magnetization direction of the first ferromagnetic layer 121 is controlled by longitudinal bias magnetic field which is produced by the hard magnets 112. When an external magnetic field applied onto the MR read head is strong enough to compensate the longitudinal bias magnetic field, the magnetization direction of the first ferromagnetic layer 121 is free to rotate in response to the external applied magnetic field, thus the first ferromagnetic layer 121 is also called “free layer” 121. The direction of the magnetization in the free layer 121 changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer 122, and hence the characteristics are obtained.
In MR read head, strength and intensity of the longitudinal bias field will impact noise generated by fluctuations or displacements of the magnetic domain boundaries. If the longitudinal bias field applied to the free layer is small, noise will be easily occurred; if the longitudinal bias field is large, the change in the magnetization direction of the free layer becomes difficult causing the sensing sensitivity of MR read head to be degraded. Thus a testing magnetic field with the same direction of the longitudinal bias field is hard to control, and the testing accuracy resulted by this testing method is low and the efficiency is low.
Hence, it is desired to provide an improved testing method of a magnetic head and a testing apparatus thereof, to overcome the above-mentioned drawbacks.